1. Field of the Invention
This invention relates to the field of coal conversion to form hydrocarbon liquids suitable for conversion to fuels.
More particularly, this invention relates to an apparatus for reacting pulverized coal with heated hydrogen to form hydrocarbon liquids suitable for conversion to fuels or for use as a chemical feedstock.
2. Description of the Prior Art
The problem is to react coal directly with hydrogen in such a way as to maximize the yield of liquid products. A number of researchers have shown that at the beginning of coal pyrolysis a transient period exists for a few tenths of a second where the coal is highly reactive toward hydrogen. If excess hydrogen is not available during this period, some of the free-radical pyrolytic fragments will strip molecular hydrogen from the aromatic groups while other fragments will polymerize to form unreactive char. The overall effect is a limited yield of liquid and gaseous hydrocarbons, and a large yield of char. If instead, excess hydrogen is present during the critical transient period, many more hydrogenated fragments that are amenable to still further hydrogenation are produced. The overall effect of pyrolysis in hydrogen is a much larger yield of liquids and gases, and a lower char yield.
It is generally well known the conversion of coal to liquid or gaseous fuels is achieved by the addition of hydrogen. This may be accomplished by the direct contact of coal with hydrogen as in the Bureau of Mines Hydrane process to produce methane; by a catalyzed liquid-phase reaction with hydrogen to produce liquid products as in the Synthoil process; or indirectly by reacting coal with steam. Many different processes have been proposed and are under development. These schemes vary in the method of contacting coal and hydrogen or steam, and in the type of coal feed utilized. A solid such as coal can be contacted with a gas in three basically different ways. In the first, gas is forced through a fixed or slowly moving bed of solid. Another method of contact is by use of a fluidized bed. With sufficiently small solid particles and a sufficiently high gas velocity in vertical upward flow the air dynamic drag forces on the individual particles begin to approach the gravitational forces and the particles themselves begin to move about. The bulk properties of the gas solid mixture then become those of a fluid. Because of the improved heat and mass transfer characteristics in a fluidized bed as opposed to a fixed bed, most coal gasification processes now are the fluidized bed variety. Yet another basic category of gas solid contacting is entrained flow as in the Bigas process. In this regime gas velocities are high enough and particle sizes low enough that the solid particles are carried along with the gas stream. An advantage of the entrained flow processes is the ability to utilize any grade or class of coal. Caking coals will agglomerate causing difficult problems when fed to fluidized or fixed bed systems. Further advantages of entrained flow with respect to gas production include operation at high temperatures so that tar production is kept to a minimum, adaptability to slagging conditions and high energy production per unit volume. The present invention utilizes this type of entrained flow coal conversion process. Heretofore no large scale attempt to use this approach for direct hydrogenation of coal has been made.
A patent issued to W. C. Schroeder, U.S. Pat. No. 3,030,297, describes a process which comprises heating dry particles of coal entrained in a heated stream of hydrogen at total pressure of about 500-6000 psig from a temperature below about 300.degree. C. to a reaction temperature in the range of from about 600.degree. C. to about 1000.degree. C. Two minutes are required to heat the coal particles to about 600.degree. C. and then two to twenty seconds time at temperature for hydrogenation. The slow heat-up results from the main hydrogen stream being utilized to carry the coal into the reactor. The products of reaction are then cooled below reaction temperature to provide a product comprised of light oil, predominantly aromatic in nature, and hydrocarbon gases, primarily methane and ethane, and carbon monoxide.
This process is disadvantaged in that the coal particles entrained in the hydrogen are preheated prior to introduction into a heating chamber thus the reaction process is started upstream of the reaction chamber which will cause agglomeration and plugging within the conduit carrying the entrained coal. The present invention overcomes this agglomeration problem by providing two sources of gas, one source of gas such as hydrogen brings entrained coal into an injector at a temperature substantially less than the coal reaction temperature, and a separate source provides heated hydrogen to an injector which contacts the entrained dense phase coal downstream of an injector within a reaction zone thereby starting the hydrogenation process within the reaction chamber and not upstream of the chamber.
Schroeder is further disadvantaged in that he attempts to heat the entrained coal particles through a tube wall. At the mass throughputs specified in the example, it is doubtful that enough heat could be transferred through the tube wall in a reasonable length to sufficiently heat the coal and, at the same time, use the tube wall to contain the system pressure. This type of reactor does not scale to the necessary larger diameters for commercial coal conversion reasonably because the heat transfer surface-to-volume ratio decreases rapidly with an increase in size.
Schroeder is still further disadvantaged in that the mixing and the heating takes place in minutes and seconds whereas the present invention accomplishes the hydrogenation of the entrained coal in milliseconds and if a uniform flow pattern can be maintained (to avoid back mixing which will cause longer residence time and gas production instead of liquids) and if the coal can be dispersed uniformly even on a microscopic scale (to minimize gas diffusion limitations), and if rapid and efficient quenching can be achieved (Schroeder carries the hydrogenated products through a conduit towards a separate quenching chamber whereas the present invention quenches the reaction products immediately upon exiting the end of the reaction chamber), then it should be possible to hydrogenate a substantial fraction of the coal to liquid products. The utilization of rocket engine type injector principles in a coal liquefaction plant as described in the present invention is believed to be unique and is one of the principal objects of the invention.
Another patent issued to Schroeder, et al., U.S. Pat. No. 3,152,063, teaches a process which comprises dispersing pulverized and catalyzed coal, in the absence of a pasting oil, in hydrogen under a pressure of about 500 to 4000 psig, reacting the mixture of coal and hydrogen at a temperature in the range of about 450.degree. C. to 600.degree. C., for a gas residence time of less than about 200 seconds, cooling the reaction products and recovering liquid and gas hydrocarbon products therefrom.
Schroeder teaches passing of catalyzed coal and hydrogen into a two-stage reactor that consists of a multiplicity of parallel tubes axially extending within the reactor. The tubes are heated by a source of hot gas to start the reaction within the tubes. Vaporized oil and gas products are drawn off as well as unused hydrogen to a cooling device. The residual heavier oil and tar products are collected in the bottom of the reactor and a source of hydrogen may then be brought in to further hydrogenate these heavier products.
This invention is disadvantaged in that the pulverized coal must be passed through a catalyzing process, sent through a dryer and grinder and finally separated into minute particles by passing the coal through a screening process. The present invention utilizes finely-divided pulverized coal directly without the foregoing pre-treatment process.
Schroeder's invention is further disadvantaged in that it also utilizes the carrier hydrogen in the coal passages as the main source of hydrogen. The heat-up process then takes considerable time as compared to the present invention in that the carrier gas cannot be pre-heated prior to entering into a reaction chamber.
Additionally, the invention is disadvantaged in that the coal particles are heated through a tube or a series of tubes thereby seriously affecting the ability to scale-up the process to commercial production proportions. A commercial unit would necessarily have to process in the neighborhood of 1000 tons/hour. The Schroeder patents teach a mass throughput of approximately 145 lbs/hr ft..sup.2, a very low process rate. For example, in a commercial reactor using the Schroeder process, each reactor being 15 feet in diameter, 82 reactors would be needed to process 1000 tons/hr of coal. In addition, because of the small surface-to-volume ratio the reactors would have to be on the order of one hundred feet long to transfer sufficient heat through the wall transporting the entrained coal particles. One of the most important advantages of the high throughput of dense phase coal particles through the reactor of the present invention (33,000 lbs/hr ft..sup.2) is that it is scaleable to a commercial size. Two reactors utilizing the principles set forth in the following specifications, 6-feet in diameter would process 1000 tons/hr of coal. The heat is supplied directly in the hydrogen so that vessel surface-to-volume ratio is not a limiting factor.
Although the chemistry of coal pyrolysis and hydrogenation has been apparent for some time, no well-developed reactor exists which efficiently utilizes the rapid-reaction regime. Some of the basic reasons for this appear to be a lack of adequate gas/solid injection and mixing technology, difficulty in meeting chemistry and residence time requirements, and agglomeration and plugging of the reactor. Hydrogenation of raw bituminous coal usually results in agglomeration, so that typical fluidized bed or moving bed reactors cannot be used as heretofore described. In addition, the requirement of short residence time (less than 1 sec) necessarily restricts the reactor to an entrained flow type. By maintaining rapid mixing, heat-up, and reaction of the coal near the point of injection and hot reactor walls, the agglomeration problem can be avoided. The uniform and precise mixing of extremely large feed streams in time of a few milliseconds is the special accomplishment of large rocket engine injectors and one of the principal objects of the present invention.